A ground fault circuit breaker is an electrical protection device that disconnects a circuit when current leakage is detected. Leakage occurs when current flowing through a line, or “hot,” conductor from a source to a load is diverted to ground without returning to the source. This leakage may result from an accidental short circuit, perhaps from a defective load attached to the line. If a person touches the load, the leakage current may pass that person's body to ground, leading to an electrical shock. Consequently, ground fault circuit breakers (“GFCIs”) act as safety devices and are designed to detect line-to-ground shorts and to disconnect the distribution circuit.
GFCIs also need to act quickly. While a typical circuit breaker interrupts the circuit at 20 amperes, it takes only about 100 milliamperes to electrocute a person. Therefore, for added safety, GFCIs must be able to detect current flow between a line and ground at current levels as little as 5 milliamperes and trip a breaker at the receptacle or at the breaker panel to remove the shock hazard. GFCIs are generally required for receptacles in bathrooms and other areas exposed to water in an effort to prevent deadly ground fault situations from occurring.
In two-line systems, GFCIs typically detect current leakage by comparing the current flowing in the line and returning in the neutral. A difference in current levels implies that some current has leaked from the circuit to ground and that a fault exists. GFCIs typically use a differential transformer to detect a difference in the line and neutral current levels. The differential transformer is often a toroidal core that has as its primary windings the line and neutral conductors of the distribution circuit being protected, which are encircled by the core. Its secondary windings are wrapped around the core.
During normal conditions, the current flowing in one direction through the line conductor will return in the opposite direction through the neutral conductor. This balance produces a net current flow of zero through the differential transformer, and the multi-turn winding provides no output.
If a fault exists, current leaks from the line conductor to ground, and the current flowing back through the line and neutral currents will not be equal through the differential transformer. This current imbalance will produce uncancelled flux in the differential transformer's core, resulting in an output from the multi-turn secondary winding. Detection circuitry identifies the output and opens the circuit breaker contacts.
A GFCI device must account for decreased sensitivity that the differential transformer may have if a ground-to-neutral fault also exists. A wiring error at the breaker or load may create a ground-to-neutral short or path. A ground-to-neutral path in itself does not create a shock hazard. However, with such a condition, when a line-to-ground fault occurs, possibly though a person's body, a portion of the current may return to the neutral conductor via the ground-to-neutral path. Another portion of the leaked current may return to the source via ground. To the extent a portion of the leaked current returns to the neutral conductor, it will escape detection by the differential transformer. In short, the current differential detected by the transformer will be lower than the actual current leaked to ground. This offset may cause the transformer to not detect a hazardous ground fault and, at the least, will cause the transformer to be less sensitive to ground faults than intended.
An example may help explain the problem. If a line-to-ground short exists, it may cause 6 milliamperes to pass from the line conductor through a human body. If 5 of the 6 milliamperes return to (or leak back onto) the neutral conductor via a ground-to-neutral fault (perhaps due to a ground wiring error), the differential transformer will only detect 1 milliamperes in leakage current. This amount may not be sufficient to trip the circuit breaker, even though the 6 milliampere leakage is hazardous.
To protect against this loss of sensitivity, GFCIs often include a second transformer, referred to as a ground-neutral transformer. The ground-neutral transformer typically comprises a solenoidal or toroidal core that encircles at least the neutral conductor and has a multi-turn winding wound thereon. Preferably, the ground-neutral transformer encircles the line conductor as well. An oscillator is generally coupled to the coils of the ground-neutral transformer to energize the coils. A common type of ground-neutral transformer is called a dormant oscillator. The ground-neutral transformer helps detect ground-to-neutral shorts and compensates for the loss of sensitivity that may arise with the differential transformer.
Most GFCIs have a “test” button for verifying the heath of the device. Current test methods generally create a small current imbalance by passing current through the core of the differential transformer. For example, pressing the test button may cause the 120 volt supply to be drawn across a 14.75 K resistor along a test wire that passes through the differential transformer. In this example, a current of 8.2 mA (milliamperes), which is greater than the 5 mA leakage current detection requirement for GFCI circuits, passes through the differential transformer. The differential transformer and detection circuitry in a correctly working device would detect the test current as an imbalance, and cause the circuit to trip. The tester interprets this result as meaning the circuit breaker device is working safely and correctly. If the circuit breaker does not trip, the tester may assume the circuit has a problem that may be dangerous and require a specialist's attention. Some GFCI devices include a reset button for resetting the breaker after it has tripped.
Unfortunately, existing GFCIs (and other protection devices that contain GFCIs, such as arc-fault circuit interrupters) only test the differential transformer portion of the circuit. No method appears to exist for testing the ground-neutral transformer. One explanation is that it has been believed that such testing may require passing a current along the ground connection, which could create a hazardous situation.
Because the ground-neutral transformer is currently not tested, a GFCI (or other ground fault circuit breaker) device may test successfully even though the circuitry for detecting ground-to-neutral faults is inoperative, giving the tester a false sense of security. A potentially dangerous (or even lethal) ground-to-neutral fault condition may exist, but there is currently no way for the tester to know.
Accordingly, in an effort to reduce the negative effects of undetected ground-to-neutral circuit faults and better facilitate testing the circuitry used for detecting these faults, systems and methods for testing ground fault detection circuitry may be required.